Laser Surface Treatment of Aerospace Steels
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LASER SURFACE TREATMENT OF AEROSPACE STEELS G. CHRISTODOULOU, P. HENRY, W.M. STEEN AND D.R.F. WEST Department of Metallurgy & Materials Science, Imperial College, London, England ABSTRACT An investigation is reported of the laser surface melting of four alloy steels with carbon contents ranging up to 1 wt%. Studies were made of the profiles, microstructures and hardnesses of zones produced using a laser power of 1.7 kW and a range of traverse speeds and beam diameters. The observed zone profiles were correlated with calculations from a three-dimensional, moving source, heat transfer model. In the high carbon steels, the shallow laser melted zones contained substantial amounts of retained austenite. INTRODUCTION The technique of laser surface melting has attracted considerable interest with the aim of modifying structures, and hence properties in many alloys including steels eg Refs. [1, 2]. Detailed knowledge is required of the relationship between laser processing parameters (eg traverse speed and beam diameter) and the shapes and microstructures of the melted zones. The present investigation explores these aspects for four steels used in aerospace applications; processing conditions ranged from relatively deep melting to transformation hardening, with no melting. EXPERIMENTAL PROCEDURE The compositions of the steels are as follows (wt%): Steel
C
S135 300M S99 S82*
1.0 0.43 0.37 0.14
Si
Mn
Ni
Cr
0.29 1.57 0.21 0.23
0.42 0.88 0.68 0.35
0.17 1.90 2.46 4.03
1.39 0.80 0.69 1.09
V
0.02 0.70 -
Mo
Austen- Tempering itizing
- 844 0C 0.35 876 0C 0.61 839 0 C 0.26 790 0C
165°C (lhr) 300'C (2+2hr) 500 0C (lh0 140 or 100 0 C (4hr)
The S and P contents were in the range 0.005-0.022 and 0.005-0.009% respectively. *S82 was carburized to a depth of -1mm The first three steels were laser treated in two conditions: (1)austenitized, OQ and (2) austenitized (1 hr) OQ + tempered AC. S82 had been autenitized, OQ and tempered after carburizing. -
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Laser treatments were given to DAG-graphite coated surfaces. _The laser power was 1.72 kW with traverse speeds ranging from 8 to 1200mm s and beam diameters from 0.4mm (focus) to 4.3mm. Microstructural observations and microhardness measurements (100g load) were made on sections of the zones. RESULTS AND DISCUSSION Shapes of laser melted zones Figs. la and b illustrate (for steel S82) the two main shapes of melted zone ie "keyhole" and "conduction limited", which were observed in all the steels [3]. The heat affected zone (which clearly reveals a segregation "banding" effect) is outlined by a narrow dark-etching region. Fig. Ic shows the limiting case where no detectable melting has occurred. Fig. 2 shows the operating regions for which these cross-section shapes are developed. Keyhole formation involves significant evaporation in contrast to the conduction limited case [3, 4]. Fig. 3a shows the zone for a small beam diameter which approximates to a point heat source; the zone section shape is approximately a segment of a circle (although incipient keyholing had oc
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